Variable resistor



Dec. 31, 1940. N; o. CLARK 2,226,846

VARIABLE RESISTOR Filed Sept. 29, 1957 29 sow' 'i 2s 3 INVENTOR Nelson 0. Clark wxw ATTORNEY Patented Dec. 31, 1940 PATENT OFFICE VARIABLE RESISTOR Nelson 0. Clark, Winthrop, Mass., assignor to Minneapolis-Honeywell Regulator Company,

Minneapolis, Minn., warea corporation of Dela- Application September 29, 1937, Serial No. 166,374'

5 Claims.

The present invention relates to a variable resistor and more particularly to one which is adapted to be changed in accordance with the value of a condition.

An object of the present invention is to provide an improved variable resistance device in which a resistance element of a material which, when placed in a magnetic field, has its resistance varied in accordance with the intensity of said field, is placedin a magnetic field with provision for varying the intensity of the field.

A further object of the present invention is to provide an improved variable resistance device of the type set forth in the previous object in which the means for varying the intensity of the field 1s a condition responsive one and varies it in accordance with the value of the condition.

A further object of the invention is to provide such a device in which the condition responsive means is a thermo-couple which energizes an electromagnet to produce the magnetic field.

A still further object of the present invention is to provide an arrangement according to the preceding objects in which provision is made for providing flux of high density adjacent the resistance element.

Other objects of the present invention will be apparent from a consideration of the accompanying specification, claims and drawing, of Which-.

Figure 1 a sectional view of the improved 7 variable resistance device of the present invention, the section being taken along the line l| of Figure 2, and 2 Figure 2 is a schematic view of a systememploying the varying resistance device and in which this device is shown in section; these'ct'ion bein'g'taken along the line 2 -2 of Figure l."

l'Refe'rririg to the drawinga core member'bf magneticmaterial is generally indicated by the reference-numeral 0." This core member "is cup shaped inform and is' provide'd with a base p'qrportion I I 3. f Surrounding the' -"ceiitral portion l3 is .an="electromagnetie c011: |4.-:- AlsJ forming a part of 1 the" magnetic core is .a' (disc member l5, 1 which is inverted cup-shaped inform-t Themember 15' and the core. lliareboth'lof materials of high magnetic permeability and. are joined togetheratlfi in any-suitable manner to insure as high magnetic conductivity-across the joint' as possible. The upper edge of annular gwalL l2 is tapered as at 8..; Similarly; the lower. annular portion of member '|5 is1tapered"a$1at 20'.

55 .sLocatedin'thespaceybetween the tapered edges of wall l2 and member I5 is a resistance element 2|, which resistance element is formed of a material having the property that when it is placed in the magnetic field its resistance is increased in accordance with the intensity of the field. Material which exhibits this property to a great extent is bismuth although some other metals, such as nickel, exhibit this property to a somewhat lesser extent. As more clearly indicated in Figure 2, the resistance element 2| is formed of an insulated wire which is doubled upon itself at 23 so as to form two oppositely wound loops. Due to'the doubling back of the wire upon itself, any inductive effect is entirely eliminated. The two ends of the wire are secured to terminal members 25 and 26.

By reason of the bismuth element 2| being located between the tapered edges of the magnetic members l2 and I5, it is subjected to a magnetic field of very high intensity. Due to the extremely high intensity of the field, it is necessary to reduce to a minimum the possibility of leakage around the bismuth element 2|. Consequently, the space between member I5and coil I4 is filled with a material of very low magnetic susceptibility. This material should not only have a low magnetic susceptibility but a very high heat conductivity and a relatively. low melting point. The latter two requirements are necessary in view of the low melting point of bismuth making it necessary to have a material of lower melting point than bismuth and at the same time a material which has sufficient heat conductivity to carry away any heat generated in the bismuth conductors; A metal which is particularly desir-' able for this purpose is ordinary solderconsisting of approximately per cent tin and 50 per cent lead. view of the necessity'of conducting away the heat, thespacebetween member l5 "and coil I4 is made relativelyflargei to: permit arelativelylarge amount of solder to be passed. I'his "solder has been indicatedin' the drawing b'yj the reference numeral28g For the sani ejpurp'oses, that "is, preventing anyfleakag'e around the elemeritZI and" 'con'ducting tl'ieheat away 'fronrthet element, an annulanmember 2E] of triangular cross section is cast on theoutside of members I5 and I0. By reason of-members 28 and =29',the

2 heat is conducted in both directions from .mem-

ber "2|,. whi1e,,-at theg'same time the. leakage,"

around this memberisheld'at averylow value so as to intensify the field to which theelement 2 Us subjected. The solder '28" a is supplied-through one of two circular openings 30 and '3| extended through. member |5."*One of these openings serves for the introduction of the other for the escape of the air.

A thermo-couple 35 is provided for energizing the electromagnetic coil l4. This thermo-couple consists of inner and outer members 32 and 33 which are of suitable thermo-electric characteristics. Inasmuch as it is desirable to subject the thermo-couple to relatively high temperatures, these elements, particularly the outer one 33, should be of a metal capable of withstanding very high temperatures. It has been found desirable to make the inner metal of Constantan, an alloy consisting of'about 50 per cent nickel and 50 per cent copper and the outer member of stainless steel. The stainless steel may be of any of solder and the .the usual alloys known by this name. The two elements 32 and 33 are joined at 34 to provide the hot junction of the thermocouple. At their inner ends the two elements are connected to opposite ends of the winding Hi. If the distance between the coil and the hot junction is of appreciable length, it is desirable to terminate the elements 32 and 33 after a relatively short distance and connect these elements with copper elements which have a considerable lower resistance. The elements 32 and 33 are separated from each other by insulation 36.

It will be readily seen that when the hot junc; tion 34 of the thermo-couple 35 is heated, the electromagnetic winding M will be energized. This is caused to produce flux in the core I 8 and the member |5. In view of the fact that the members l8 and 5 are tapered at their adjacent edges between which the resistance element 2| is located, this resistance element is subjected to flux of extremely high density. The efl'ect of thistfiux is to change the resistance of element 2|. It will be readily apparent that the strength of the magnetic field and consequently the value of resistance 2| will be dependent upon the temperature of the hot junction 34. Accordingly, the arrangement provides a means for varying the value of the resistance in accordance with the value of the controlling temperature. The device has the advantage that it can measure extremely high temperatures since the thermocouple 35 can be placed in the region of high temperatures and the rest of the structure can be placed outside thereof.

While it is obvious that the resistance device just described can be employed for a variety of purposes, such a device is particularly adapted for use in condition control systems. Such an flow of fuel to a burner, or other similar heating device. In this figure, the valve has been generally indicated by the reference-numeral 48. This valve is provided with a valve stem 4|. The valve 40 controls the flow of fuel through a fuel line 42 leading to a burner, or other similarheating device (not shown). A motor generally indicated by the reference numeral 45 controls the position of valve 48. The motor 45 is of the condenser induction type and comprises ,a rotor. and a pair of field windings 41 and 48. Associated with the field windings is a condenser 49. The rotor 46 is connected to a reduction gear train 58 and drives, through the reduction gear train 58, -a shaft 5|. Shaft 5| carries a crank disc 52 which is connected'through a link 53 with the valve the two field windings 41 and48. When the condenser is connected in series with one of the field windings, the rotor rotates in one direction, and

when connected. in series with the other field winding, it rotat'es in the other direction.

A relay generally indicated by the reference numeral 55 controls the energization of the motor and the connection of the condenser 49 to the field windings 41 and 48. The relay consists of a pair of field coils 56 and 51 which are connected together at one end. Associated with the coils 56 and 51 is an armature member 58 which is operatively connected through a suitable connecting member 59 with a switch blade 60. The switch blade 60 is adapted to engage either of two contacts 6| and 62. When relay coil 56 is more highly energized than relay coil 51, armature 58 isdrawn to the left moving the switch blade 60 into engagement with contact 6|. When, on the other hand, relay coil 51 is more highly energized than relay coil 56, the opposite movement of armature 58 takes place causing switch blade 68 to be 'moved into engagement with contact 62. v

A step-down transformer is employed for supplying low voltage power for operation of this system. This step-down transformer is generally indicated by the reference numeral 65 and comprises a low voltage secondary 66 and a line voltage primary 61. The line voltage primary 61 is connected to line wires 69 and 10 leading to any suitable source of power (not shown). The low voltage secondary 66 is connected to conductors1l, 12, 13, and 14 to the opposite terminals of the relay coils 56 and 51. Accordingly, it will be seen that the relay coils 56 and 51 are connected in series across the secondary. The terminal 26 of the two terminals 25 and 26 ofthe variable resistance device previously described is connected through a conductor 16 with the outer terminal of relay coil 56. The other terminal 25 of the variable resistance device is connected through conductors 11 and 18 to the movable contact 19 of a rheostat 88, the fixed resistance 8| of which is connected through conductors 82 and 83 to the junction of relay coils 56 and 51. It will accordingly be apparent that the variable resistance element 2| is connected across the relay coil 56 in-series with the rheostat 88. For any fixed adjustment of the rheostat 80, the voltage impressed across the relay coil 56 is controlled in part by the value of resistance 2|.

A second rheostat 85 is provided for the purpose of controlling the energization of relay coil 51. This rheostat 85 comprises a. resistance 86 and a movable contact 81. The resistance 86 is connected directly to the left-hand terminal of relay coil 51 and the movable contact 81 is connected through conductors 88 and 18 with the movable contact 19 of rheostat '88. It will ac cordingly be seen that rheostat 85 is connected across relay coil 51 in series with the rheostat 88. The rheostat 88 is accordingly in circuit with boththe resistance 2| and the rheostat 85. The rheostat 85 is used for adjusting the temperature setting of the variable resistance including resistance element 2|. It will be apparent that the point at which the energization'of resistance elements 56 and 51 is balanced is determined by the setting of the rheostat 85. By moving the slider 81 to the right, it is necessary to have a considerably higher temperature before the resistance element 2| has a resistance equal to that of the-rheostat 85. Similarly, moving the slider the resistance value of resistance 2| will haveupon the relative energization of relay coils 56 and 51.

For a given setting of rheostats 89 and 85, it will be apparent that an increase in the temperature to which thermo-couple 35 is subjected will cause relay coil 56 to have a higher voltage impressed across the same and consequently to become more highly energized than relay coil 51. Similarly, a decrease 'in the temperature to which thermo-couple 35 is subjected will cause a decrease in the energization of relay coil 56.

A potentiometer 99 is used for also controlling the relative energization of relay coils 56 and 51. This potentiometer comprises a resistance ele- .ment 9| and a movable contact arm 92. The movable contact arm 92 is secured to shaft 5| so that the same is actuated whenever the position of the valve is changed. The opposite terminals of resistance 9| are connected through conductors 94 and 12 on the one hand and 95 and 14 on the other hand to the opposite terminals of relay coils 56 and 51. The arm 92 is connected through conductors 96 and 83 with the junction of relay coils 56 and 51. It will be readily apparent that the potentiometer 99 acts as a voltage divider and that the position of contact arm 92 controls the relative energizations of relay coils 56 and 51. Thus, any movement of contact arm 92 to the right tends to increase the energization of relay coil 56 anddecrease that of relay coil 51. Conversely, when the arm 92 is moved 49 to the left, the energization of relay coil 51 is increased and that of relay coil 56 is decreased. The potentiometer 99 is used as a rebalancing potentiometer to terminate movement of the motor after the same has moved a distance cor- 49 responding to the temperature change producing the initial unbalance in the energization of relay coils 56 and 51. parent from a description of the operation of the system as a whole.

Operation of the system The Various elements of the system are shown in the position which they occupy when the temperature to which thermo-couple 35 is subjected is approximately at the desired value. Let it now be assumed that the temperature to which thermo-couple 35 is subjected decreases in value. 4 This will cause a decrease in the flux generated by the electromagnetic coil I4 and consequently a decrease in the resistance value of resistance element 2|. The result is that the energization of relay coil 56 will be decreased relative to that of relay coil 51 causing armature 58 to move to the right and move switch blade 69 into engagement with contact 62. When this takes place, the following circuit is established to the field winding 41: from line wire 69 through conductor 96, switch arm 69, contact 62, conductors 99 and I99, field winding 41, and conductor |9| to the other line wire 19. At the same time, a circuit is established to the other field winding 48 as follows: from line wire 69 through conductor 98, switch arm 69, contact 62, conductors 99 and I93, condenser '49, conductors I94 and I95, field wind- 75 ing 46, and conductor |9| to the other line wire resistance 2| to correspondingly increase.

This operation will be more ap- 1 ing 41 as follows: from line wire 69 through conv 19. It will be noted that field winding 41 is directly connected across the line wires whereas field winding 48 is connected in series with condenser 49. The result is that the current through field winding48 leads that through field winding 41 so as to produce a rotating field which causes rotation of rotor 46. The rotation of rotor 46 causes a clockwise movement of crank disc 52 to move valve 49 towards open position. This admits more fuel to the burner, or other fuel consuming device, 'to raise the temperature to which thermo-couple 35 is subjected.

At the same time that the rotation of crank disc 52 is being efiected, contact arm 92 is rotated towards the right on resistance 9|. As previously explained, movement of contact arm 92 to the right on resistance 9| tends to increase the energization of relay coil 56 and decrease that of relay coil 51. It will be obvious that this action is opposite to that produced by the decrease in the temperature to which thermo-couple 35 is subjected so that after movement of contact arm 92 has continued for a certain time, energization of relay coils 56 and 51 will again be balanced so that armature 58 moves the switch blade 69 away from contact 62 into the neutral position shown in the drawing. This will terminate operation of the motor, the motor having moved the valve a distance corresponding to the temperature change causing the original unbalance in energization of relay coils 56 and 51.

Let it be assumed now that the additional fuel supplied by valve 49 causes the temperature adjacent thermo couple 35 to rise again, the increase in temperature to which thermo-couple 35 is subjected will cause the resistance value of The effect of this is to increase the energization of relay coil 56 and to unbalance the relay 55 in the opposite direction causing switch blade 69 to be moved into engagement with contact 6|. When this occurs, the following circuit is established to relay coil 48: from the line wire 69 through conductor 96, switch blade 69,-contact 6|, con-- ductors I91 and I95, field winding 46, and conductor |9| to the other line wire 19. At the same time, a circuit is established to field windis directly connected across the source of power whereas field winding 41 is connected in series with condenser 49. The current through field winding 41 accordingly now leads that through field winding 49 so as to cause rotation of rotor 46 in a direction opposite to that which previously occurred. This opposite rotation of rotor 46 will cause a counter-clockwise rotation of shaft 5| and consequently a counter-clockwise movement of crank disc 52. This movement of crank disc 52 is effective to move valve 49 towards closed position reducing the amount of fuel supplied to the burner, 'or other heating device.

' At the same time that crank disc 52 is rotated in a counter-clockwise direction, the contact arm 92 is moved to the left on resistance 9|. This has as its efiect the increase in the energization of relay coil 51 and a decrease in the energization of relay coil 56. It will be apparent that after a limited amount of movement of the shaft 5|, the energization of relay coils 56 and 51 will again be rebalanced causing switch blade 69 to the exception of a narrow annular gap, a resist- .move to the position shown in the drawing, in

which position the operation of the motor is terminated.

It will be apparent from the foregoing description that the position of valve 40 is controlled at all times by the temperature to which thermo-couple 35 is subjected. The use of the variable resistance device of the present invention eliminates any necessity for movable contact members on a condition responsive device. This is particularly desirable in the case of a condition responsive variable resistance since the power which a condition responsive element is capable of delivering without too great sluggishness or inaccuracy is extremely limited. In a system such as that shown in Figure 2, it is possible to employ movable contact members in the rebalancing means since this is driven by a relatively powerful motor. However, it is not possible in the main control resistance for the reason pointed out above. The effect, therefore, of the substitution of the variable resistance means of the present invention is to greatly increase the accuracy of the system as a whole.

While I have shown the variable resistance device of the present invention as applied to a proportioning system for the control of a temperature controlling valve, it is to be understood that the same is adapted for numerous other applications. In general while I have shown a specific embodiment of my invention, it is to be understood that this is for purposes of illustration only and that the invention is to be limited only by the scope of the appended claims.

I claim as my invention:

1. A variable resistance arrangement comprising a core of magnetic material including a central portion and an outer cylindrical shell forming with said central portion a closed magnetic path except for a narrow annular gap, an electromagnetic coil within said outer shell and surrounding said central portion, a circular resistance element'in said gap longitudinally thereof and of a material which when placed in a magnetic field has its resistance varied in accordance with the intensity of the field, and means for variably energizing said coil.

2. A variable resistance arrangement including an electromagnetic coil, a core of magnetic material on which said coil is disposed, said core forming a completely closed magnetic path with ance element in said gap and of a material which when placed in a magnetic field has its resistance varied in accordance with the intensity of the field, material of high heat conductivity and low magnetic susceptibility disposed on each side of said gap and extending over the adjacent magnetic material, and means for variably energizing said coil.

3. A variable resistance arrangement comprising an electromagnetic coil, a core of magnetic material on which said coil is disposed, said core forming a completely closed magnetic path with the exception of a narrow annular gap adjacent which the core is tapered to concentrate the flux, a resistance element in said gap and of a material which when placed in a magnetic field has its resistance varied in accordance with the intensity of the field, metal of high heat conductivity and low magnetic susceptibility disposed on each side of said gap and extending over the tapered portions of the magnetic core, and means for variably energizingsaid coil.

4. A variable resistance arrangement including an electromagnetic coil, a core of magnetic material on which said coil is disposed, said core forming a completely closed magnetic path with the exception of a narrow elongated gap, a resistance element in said gap longitudinally thereof and of a material which when placed in a magnetic field has its resistance varied in accordance with the intensity of the field, material of high heat conductivity and low magnetic susceptibility disposed on each side of said gap closely adjacent said element, and means for variably energizing said coil.

5. A variable resistance arrangement including an electromagnetic coil, a core of magnetic material on which said coil is disposed, said core forming a completely closed magnetic path with the exception of a narrow elongated gap, a resistance element in said gap longitudinally thereof and of a material which when placed in a magnetic field has its resistance varied in accordance with the intensity of the field, material of high heat conductivity and low magnetic susceptibility disposed on each side of said gap and extending over the adjacent magnetic material, and means for variably energizing said coil. 

